Historically, partitioning of biomarkers from blood to sweat has been demonstrated in great detail. As more of these biomarkers emerge, sweat appears to be a convincing media for continuous and spontaneous health monitoring. However, ‘clinical’ techniques of sample collection, purification and analysis have restricted growth in the sweat-sensing area because of the cost and time associated with these techniques. With the advent of miniaturized sensors, however, many of these issues can be alleviated. Still, a large task has largely been left unexplored for compact sweat sensing technologies: sample extraction and collection.
Common techniques for sweat stimulation and analysis involve sweat stimulation in a region from a sweat generating unit 10, followed by removal of this unit 10 from the skin 12, cleaning of the skin 12 and reapplication of a sensing unit 14 or collection device 16, as shown in FIGS. 1A-1C. Often, the sensing unit 14 has integrated communication protocol to alert the user. This communication method could be via wireless or wired connections. For example, cystic fibrosis testing often involves this technique as demonstrated by ELITechGroup® in their Macroduct® and Nanoduct® products. This technique is problematic because it requires a two-step process that is inconvenient, non-continuous, and where reproducibility could be difficult.
Further, contamination from the stimulation reservoir and sensor region is unavoidable as they share the same area. In reference to FIGS. 1A-1C, the sweat generation unit 10 has potential to alter the state of the skin 12, whether that includes excessive hydration, thermal heating, irritation, pharmacological side-effects, or another side effect. This adds a confounding factor to sensing sweat analytes when the sensing unit 14 or collection device 16 is placed on the skin.
Attempts to reduce contamination have previously been made. For example, a technique includes utilizing an isolating membrane between sweat stimulation mechanisms and the sensors and sensing sites. However, such techniques utilizing isolating membranes (or similar techniques) may only partially or temporarily separate sweat stimulation mechanisms, such as an electric field and/or chemicals, from the sensors and sensing sites. In the instance of isolating membranes, these also have the drawback of increasing the dead volume between a sensor and the skin 12, which reduces temporal resolution. Furthermore, horizontal iontophoretic driving of an iontophoretic chemical, such as pilocarpine, may be used. However, this will again subject the sensor, sweat, and skin to an electric field and/or contamination. For many biomarkers and sensors, such interference could reduce performance of a sweat sensing device, in some cases making sensing impossible. There is an increasing need to provide improved sweat sensing techniques and devices that address one or more of the above drawbacks.